The effect of dry and humid hot air inhalation on expired relative humidity during exercise

It has been previously demonstrated that under certain environmental conditions, expired air is not fully water saturated because of the low relative humidity of the first part of the expirate. This finding is of interest to those involved in respirator research, particularly those who design and test robotic metabolic simulators. These simulators must accurately mimic the physiological responses of human airways to breathing air of various temperatures and relative humidities (RHs). Because these responses are not fully quantified, this study examined the mean relative humidity of expired air during four inspired air conditions: cool dry (26 degrees C, 60% RH), cool humid (26 degrees C, 95% RH), hot dry (45 degrees C, 11% RH), and hot humid (45 degrees C, 95% RH). These conditions were administered during three exercise intensities: rest, low (35% VO2max), and moderate (70% VO2max). As compared to the cool dry (CD) condition, frequency of breathing (f) was 9.3% lower and tidal volume (VT) was 9.4% greater across all exercise intensities for the hot humid (HH) condition (p less than 0.05). Mean expired relative humidity (ERH) was substantially lower for the hot dry (HD) condition as compared to the other three conditions during each sampling period. These findings support the conclusion that the mean ERH of expired air depends upon several respiratory and environmental factors in addition to inspired air temperature.